Behave J. Rosenberg
Internet-Draft Cisco Systems
Expires: April 9, 2007 R. Mahy
Plantronics
C. Huitema
Microsoft
October 6, 2006
Obtaining Relay Addresses from Simple Traversal Underneath NAT (STUN)draft-ietf-behave-turn-02
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This specification defines a usage of the Simple Traversal of UDP
Through NAT (STUN) Protocol for asking the STUN server to relay
packets towards a client. This usage is useful for elements behind
NATs whose mapping behavior is address and port dependent. The
extension purposefully restricts the ways in which the relayed
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Internet-Draft TURN October 20061. Introduction
The Simple Traversal of UDP Through NAT (STUN) [1] provides a suite
of tools for facilitating the traversal of NAT. Specifically, it
defines the Binding Request, which is used by a client to determine
its reflexive transport address towards the STUN server. The
reflexive transport address can be used by the client for receiving
packets from peers, but only when the client is behind "good" NATs.
In particular, if a client is behind a NAT whose mapping behavior
[15] is address or address and port dependent (sometimes called "bad"
NATs), the reflexive transport address will not be usable for
communicating with a peer.
The only way to obtain a transport address that can be used for
corresponding with a peer through such a NAT is to make use of a
relay. The relay sits on the public side of the NAT, and allocates
transport addresses to clients reaching it from behind the private
side of the NAT. These allocated addresses are from interfaces on
the relay. When the relay receives a packet on one of these
allocated addresses, the relay forwards it toward the client.
This specification defines a usage of STUN, called the relay usage,
that allows a client to request an address on the STUN server itself,
so that the STUN server acts as a relay. To accomplish that, this
usage defines a handful of new STUN requests and indications. The
Allocate request is the most fundamental component of this usage. It
is used to provide the client with a transport address that is
relayed through the STUN server. A transport address which relays
through an intermediary is called a relayed transport address.
Though a relayed address is highly likely to work when corresponding
with a peer, it comes at high cost to the provider of the relay
service. As a consequence, relayed transport addresses should only
be used as a last resort. Protocols using relayed transport
addresses should make use of mechanisms to dynamically determine
whether such an address is actually needed. One such mechanism,
defined for multimedia session establishment protocols, based on the
offer/answer protocol in RFC 3264 [7], is Interactive Connectivity
Establishment (ICE) [14].
The mechanism defined here was previously a standalone protocol
called Traversal Using Relay NAT (TURN), and is now defined as a
usage of STUN.
2. Terminology
In this document, the key words MUST, MUST NOT, REQUIRED, SHALL,
SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL are to
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be interpreted as described in RFC 2119 [2] and indicate requirement
levels for compliant STUN relay implementations.
3. Definitions
Relayed Transport Address: A transport address that terminates on a
server, and is forwarded towards the client. The STUN Allocate
Request can be used to obtain a relayed transport address, for
example.
STUN relay client: A STUN client that implements this specification.
It obtains a relayed transport address that it provides to a small
number of peers (usually one).
STUN relay server: A STUN server that implements this specification.
It relays data between a STUN relay client and its peer.
5-tuple: A combination of the source IP address and port, destination
IP address and port, and transport protocol (UDP, TCP, or TLS over
TCP). It uniquely identifies a TCP connection, TLS channel, or
bi-directional flow of UDP datagrams.
4. Overview of Operation
In a typical configuration, a STUN relay client is connected to a
private network and through one or more NATs to the public Internet.
On the public Internet is a STUN relay server. The STUN Relay usage
defines several new messages and a new framing mechanism that add the
ability for a STUN server to act as a packet relay. The text in this
section explains the typical usage of this relay extension.
4.1 Allocations
The client sends an Allocate request to the server, which the server
authenticates. The server generates an Allocate response with the
allocated address, port, and target transport.
A successful Allocate Request just reserves an address on the STUN
relay server. Data does not flow through an allocated port until the
STUN relay client asks the STUN relay server to open a binding by
sending data to the far end with a Send Indication. This insures
that a client can't use a STUN relay server to run a traditional
server, and partially protects the client from DoS attacks.
Once a binding is open, the client can then receive data flowing back
from its peer. Initially this data is wrapped in a STUN Data
Indication. Since multiple bindings can be open simultaneously, the
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Data Indication contains the Remote Address attribute so the STUN
relay client knows which peer sent the data. The client can send
data to any of its peers with the Send Indication.
Once the client wants to primarily receive from one peer, it can send
a SetActiveDestination request. All subsequent data received from
the active peer is forwarded directly to the client and vice versa,
except that it is wrapped or framed according to the protocol used
between the STUN relay client and STUN relay server.
When the STUN relay client to server protocol is a datagram protocol
(UDP), any datagram received from the active peer that has the STUN
magic cookie is wrapped in a Data Indication. Likewise any datagram
sent by the client that has the STUN magic cookie and is intended for
the active peer is wrapped in a Send Indication. This wrapping
prevents the STUN relay server from inappropriately interpreting end-
to-end data.
Over stream-based transports (TCP and TLS over TLS), the STUN relay
client and server always use some additional framing so that end-to-
end data is distinguishable from STUN control messages. This
additional framing just has a type and a length field. The value of
the type field was chosen so it is always distinguishable from an
unframed STUN request or response.
The SetActiveDestination Request does not close other bindings. Data
to and from other peers is still wrapped in Send and Data indications
respectively. If the client does not want to receive data from a
peer, it can also explicitly squelch data from a specific peer by
sending a CloseBinding request. A CloseBinding request leaves the
port allocated, so it can be reused. A CloseBinding request which
deletes the active destination, also unsets the active destination.
Allocations can also request specific attributes such as the desired
Lifetime of the allocation, and the maximum Bandwidth. Clients can
also request specific port assignment behavior. For example, a
specific port number, odd or even port numbers, pairs of sequential
port numbers.
4.2 Transports
STUN relay clients can communicate with a STUN relay server using
UDP, TCP, or TLS over TCP. A STUN relay can even relay traffic
between two different transports with certain restrictions. A STUN
relay can never relay from an unreliable transport (client to server)
to a reliable transport to the peer. Note that a STUN relay server
never has a TLS relationship with a client's peer, since the STUN
relay server does not interpret data above the TCP layer. When
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relaying data sent from a stream-based protocol to a UDP peer, the
STUN relay server emits datagrams which are the same length as the
length field in the STUN TCP framing or the length field in a Send
Indication. Likewise, when a UDP datagram is relayed from a peer
over a stream-based transport, the length of the datagram is the
length of the TCP framing or Data Indication.
+----------------------+--------------------+
| client to STUN relay | STUN relay to peer |
+----------------------+--------------------+
| UDP | UDP |
| TCP | TCP |
| TCP | UDP |
| TLS | TCP |
| TLS | UDP |
+----------------------+--------------------+
For STUN relay clients, using TLS over TCP provides two benefits.
When using TLS, the client can be assured that the address of the
client's peers are not visible to an attacker except by traffic
analysis downstream of the STUN relay server. Second, the client may
be able to communicate with STUN relay servers using TLS that it
would not be able to communicate with using TCP or UDP due to the
configuration of a firewall between the STUN relay client and its
server. TLS between the client and STUN relay server in this case
just facilitates traversal.
When the STUN relay-to-peer leg is TCP, the STUN relay client needs
to be aware of the status of these TCP connections. The STUN relay
extension defines application states for a TCP connection as follows:
LISTEN, ESTABLISHED, CLOSED. Consequently, the STUN relay server
sends a ConnectionState Indication for a binding whenever the relay
connection status changes for one of the client's bindings except
when the status changes due to a STUN relay client request (ex: an
explicit binding close or deallocation).
4.3 Tuple Terminology
To relay data to and from the correct location, the STUN relay server
maintains a binding between an internal address (called a 5-tuple)
and one or more external 5-tuples, as shown in Figure 1. The
internal 5-tuple identifies the path between the STUN relay client
and the STUN relay server. It consists of the protocol (UDP, TCP, or
TLS over TCP), the internal local IP address and port number and the
source IP address and port number of the STUN client, as seen by the
relay server. For example, for UDP, the internal 5-tuple is the
combination of the IP address and port from which the STUN client
sent its Allocate Request, with the IP address and port to which that
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Allocate Request was sent.
The external local transport address is the IP address and port
allocated to the STUN relay client (the allocated transport address).
The external 5-tuple is the combination of the external local
transport address and the IP address and port of an external client
that the STUN client is communicating with through the STUN server.
Initially, there aren't any external 5-tuples, since the STUN client
hasn't communicated with any other hosts yet. As packets are
received on or sent from the allocated transport address, external
5-tuples are created.
While the terminology used in this document refers to 5-tuples,
the STUN relay server can store whatever identifier it likes that
yields identical results. Specifically, many implementations may
use a file-descriptor in place of a 5-tuple to represent a TCP
connection.
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place a transport address into a protocol message, with the
expectation that the client will be able to receive packets from a
small number of hosts (typically one). Data from the peer is only
relayed to the client after the client sends packets towards the
peer. Because of this limitation, relayed transport addresses
obtained from an Allocate request are only useful when combined with
rendezvous protocols of some sort, which allow the client to discover
the addresses of the hosts it will be corresponding with. Examples
of such protocols include the Session Initiation Protocol (SIP) [6].
This limitation is purposeful. Relayed transport addresses obtained
from the Allocate request can not be used to run general purpose
servers, such as a web or email server. This means that the relay
usage can be safely permitted to pass through NATs and firewalls
without fear of compromising the purpose of having them there in the
first place. Indeed, a relayed transport address obtained from a
STUN relay has many of the properties of a transport address obtained
from a NAT whose filtering policies are address dependent, but whose
mapping properties are endpoint independent [15], and thus "good"
NATs. Indeed, to some degree, the relay turns a bad NAT into a good
NAT by, quite ironically, adding another NAT function - the relay
itself.
6. Client Discovery of Server
STUN requires all usages to define the mechanism by which a client
discovers the server [1]. This section contains that information for
the relay usage.
The relay usage differs from the other usages defined in [1] in that
it demands substantial resources from the STUN server. In addition,
it seems likely that administrators might want to block connections
from clients to the STUN server for relaying separated from
connections for the purposes of binding discovery. As a consequence,
the relay usage is defined to run on a separate port from other
usages. The client discovers the address and port of the STUN server
for the relay usage using the same DNS procedures defined in [1], but
using an SRV service name of "stun-relay" instead of just "stun".
For example, to find STUN relay servers in the example.com domain,
the STUN relay client performs a lookup for '_stun-
relay._udp.example.com', '_stun-relay._tcp.example.com', and '_stun-
relay._tls.example.com' if the STUN client wants to communicate with
the STUN relay server using UDP, TCP, or TLS over TCP, respectively.
The client assumes that all permissable transport protocols are
supported from the STUN relay server to the peer for the client to
server protocol selected.
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STUN requires all usages to define the mechanism by which the server
determines the usage [1]. This section contains that information for
the relay usage.
The STUN server is designed so the relay usage can run on a separate
source port from non-relay usages. Since the client looks up the
port number for the relay usage separately, servers can be configured
to rely on this property. The STUN server MAY accept both relay and
non-relay usages on the same port number, in which case it uses
framing hints and choice of STUN messages to detect the STUN usage in
use by a specific client. [TODO FIX]
The relay usage is defined by a specific set of requests and
indications. As a consequence, the server knows that this usage in
being used because those request and indications were used. [TODO
Add more here about TCP framing and knowing that either STUN relay
and/or other STUN could be running]
8. New Framing Mechanism for Stream-Oriented Transports
Over stream-based transports, the STUN relay client and server need
to use additional framing so that end-to-end data is distinguishable
from STUN control messages, and so that the relay server can perform
conversion from streams to datagrams and vice versa. This additional
framing has a one octet type, one reserved octet, and a 2 octet
length field. The first octet of this framing is 0x02 to indicate
STUN messages or 0x03 to indicate end-to-end data to or from the
active destination. Note that the first octet is always
distinguishable from an unframed STUN request or response (which is
always 0x00 or 0x01). The second octet is reserved and MUST be set
to zero. The length field counts the number of octets immediately
after the length field itself.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Reserved = 0 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Use of this framing mechanism is discussed in Section 12 and
Section 13.
9. New Requests and Indications
This usage defines three new requests (along with their success and
error responses) and three indications. It also defines processing
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rules for the STUN server and client on receipt of non-STUN messages.
See Section 12 and Section 13
The new messages are:
0x0003 : Allocate Request
0x0103 : Allocate Response
0x0113 : Allocate Error Response
0x0004 : Send Indication
0x0115 : Data Indication
0x0006 : Set Active Destination Request
0x0106 : Set Active Destination Response
0x0116 : Set Active Destination Error Response
0x0007 : Connect Request
0x0107 : Connect Response
0x0117 : Connect Error Response
0x0118 : Connect Status Indication
In addition to STUN Requests and Responses, STUN relay clients and
servers send and receive non-STUN packets on the same ports used for
STUN messages. How these entities distinguish STUN and non-STUN
traffic is discussed in Section 12 and Section 13.
9.1 Allocate Request9.1.1 Server Behavior
The server first processes the request according to the general
request processing rules in [1]. This includes performing
authentication and checking for mandatory unknown attributes. Due to
the fact that the STUN server is allocating resources for processing
the request, Allocate requests MUST be authenticated, and
furthermore, MUST be authenticated using either a shared secret known
between the client and server, or a short term password derived from
it.
Note that Allocate requests, like all other STUN requests, can be
sent to the STUN server over UDP, TCP, or TCP/TLS.
The behavior of the server when receiving an Allocate Request depends
on whether the request is an initial one, or a subsequent one. An
initial request is one whose source and destination transport address
do not match the internal remote and local transport addresses of an
existing internal 5-tuple. A subsequent request is one whose source
and destination transport address matches the internal remote and
local transport address of an existing internal 5-tuple.
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[[TODO: First add short summary of what are we trying to do here]]
The server attempts to allocate transport addresses. It first looks
for the BANDWIDTH attribute for the request. If present, the server
determines whether or not it has sufficient capacity to handle a
binding that will generate the requested bandwidth.
If it does, the server attempts to allocate a transport address for
the client. The Allocate request can contain several additional
attributes that allow the client to request specific characteristics
of the transport address. First, the server checks for the
REQUESTED-TRANSPORT attribute. This indicates the transport protocol
requested by the client. This specification defines values for UDP
and TCP. The server MUST allocate a port using the requested
transport protocol. If the REQUESTED-TRANSPORT attribute contains a
value of the transport protocol unknown to the server, or known to
the server but not supported by the server in the context of this
request, the server MUST reject the request and include a 442
(Unsupported Transport Protocol) in the response, or else redirect
the request. [[OPEN ISSUE: Should we include a list of supported
ones? Is this really an issue? If its just ever TCP and UDP its not
needed. Can always add it later, as the hooks are here. Proposal:
Do not incldue a list of supported transports.]]. If the request did
not contain a REQUESTED-TRANSPORT attribute, the server MUST use the
same transport protocol as the request arrived on.
As a consequence of the REQUESTED-TRANSPORT attribute, it is possible
for a client to connect to the server over TCP or TLS over TCP and
request a UDP transport address. In this case, the server will relay
data between the transports.
Next, the server checks for the REQUESTED-IP attribute. If present,
it indicates a specific interface from which the client would like
its transport address allocated. If this interface is not a valid
one for allocations on the server, the server MUST reject the request
and include a 443 (Invalid IP Address) error code in the response, or
else redirect the request to a server that is known to support this
IP address. If the IP address is one that is valid for allocations
(presumably, the server is configured to know the set of IP addresses
from which it performs allocations), the server MUST provide an
allocation from that IP address. If the attribute is not present,
the selection of an IP address is at the discretion of the server.
Finally, the server checks for the REQUESTED-PORT-PROPS attribute.
If present, it indicates specific port properties desired by the
client. This attribute is split into two portions: one portion for
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port behavior and the other for requested port alignment (whether the
allocated port is odd, even, reserved as a pair, or at the discretion
of the server).
If the port behavior requested is for a Specific Port, the server
MUST attempt to allocate that specific port for the client. If the
port is allocated to a different internal 5-tuple associated with the
same STUN long-term credentials, the client is requesting a move.
The server SHOULD replace the old internal 5-tuple with the new one
over which this Allocate request arrived. The server MUST reject the
move request if any of the attributes other than LIFETIME have
changed (BANDWIDTH, REQUESTED-TRANSPORT, etc.).
If the specific port is not available (in use or reserved), the
server MUST reject the request with a 444 (Invalid Port) response or
redirect to an alternate server. For example, the STUN server could
reject a request for a Specific Port because the port is temporarily
reserved as part of an adjacent pair of ports, or because the
requested port is a well-known port (1-1023).
If the client requests even port alignment, the server MUST attempt
to allocate an even port for the client. If an even port cannot be
obtained, the server MUST reject the request with a 444 (Invalid
Port) response or redirect to an alternate server. If the client
request odd port alignment, the server MUST attempt to allocate an
odd port for the client. If an odd port cannot be obtained, the
server MUST reject the request with a 444 (Invalid Port) response or
redirect to an alternate server. Finally, the Even port with hold of
the next higher port is similar to Even port. It is a request for an
even port, and MUST be rejected by the server if an even port cannot
be provided, or redirected to an alternate server. However, it is
also a hint from the client that the client will request the next
higher port with a separate Allocate request. As such, it is a
request for the server to allocate an even port whose next higher
port is also available, and furthermore, a request for the server to
not allocate that one higher port to any other request except for one
that asks for that port explicitly. The server can honor this
request for adjacency at its discretion. The only constraint is that
the allocated port has to be even.
Port alignment requests exist for compatibility with
implementations of RTP which pre-date RFC 3550. These
implementations use the port numbering conventions in (now
obsolete) RFC 1889.
If any of the requested or desired constraints cannot be met, whether
it be bandwidth, transport protocol, IP address or port, instead of
rejecting the request, the server can alternately redirect the client
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to a different server that may be able to fulfill the request. This
is accomplished using the 300 error response and ALTERNATE-SERVER
attribute.
The server SHOULD only allocate ports in the range 1024-65535. This
is one of several ways to prohibit relayed transport addresses from
being used to attempt to run standard services. These guidelines are
meant to be consistent with [15], since the relay is effectively a
NAT.
Once the port is allocated, the server associates it with the
internal 5-tuple and fills in that 5-tuple. The internal remote
transport address of the internal 5-tuple is set to the source
transport address of the Allocate Request. The internal local
transport address of the internal 5-tuple is set to the destination
transport address of the Allocate Request. For TCP, this amounts to
associating the TCP connection from the STUN relay client with the
allocated transport address.
If the Allocate request was authenticated using a shared secret
between the client and server, this credential MUST be associated
with the allocation. If the request was authenticated using a short
term password derived from a shared secret, that shared secret MUST
be associated with the allocation. This is used in subsequent
Allocate requests to ensure that only the same client can refresh or
modify the characteristics of the allocation it was given.
The allocation created by the Allocate request is also associated
with a transport address, called the active destination. This
transport address is used for forwarding data through the STUN relay
server, and is described in more detail later. It is initially set
to null when the allocation is created. In addition, the allocation
created by the server is associated with a set of permissions. Each
permission is a specific IP address identifying an external client.
Initially, this list is null. Send Indications, Connect requests and
Set Active Destination requests add values to this list.
If the LIFETIME attribute was present in the request, and the value
is larger than the maximum duration the server is willing to use for
the lifetime of the allocation, the server MAY lower it to that
maximum. However, the server MUST NOT increase the duration
requested in the LIFETIME attribute. If there was no LIFETIME
attribute, the server may choose a default duration at its
discretion. In either case, the resulting duration is added to the
current time, and a timer, called the allocation expiration timer, is
set to fire at or after that time. Section 13.3 discusses behavior
when the timer fires. Note that the LIFETIME attribute in the
request can be zero. This typically happens for subsequent
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Allocations, and provides a mechanism to delete the allocation. It
will force the immediate deleting of the allocation.
Once the port has been obtained from the operating system and the
activity timer started for the port binding, the server generates an
Allocate Response using the general procedures defined in [1]. The
transport address allocated to the client MUST be included in the
RELAY-ADDRESS attribute in the response. In addition, this response
MUST contain the XOR-MAPPED-ADDRESS attribute. This allows the
client to determine its reflexive transport address in addition to a
relayed transport address, from the same Allocate request.
The server MUST add a LIFETIME attribute to the Allocate Response.
This attribute contains the duration, in seconds, of the allocation
expiration timer associated with this allocation.
The server MUST add a BANDWIDTH attribute to the Allocate Response.
This MUST be equal to the attribute from the request, if one was
present. Otherwise, it indicates a per-binding cap that the server
is placing on the bandwidth usage on each binding. Such caps are
needed to prevent against denial-of-service attacks (See Section 14).
The server MUST add, as the final attribute of the request, a
MESSAGE-INTEGRITY attribute. The key used in the HMAC MUST be the
same as that used to validate the request.
If the allocated port was for TCP, the server MUST be prepared to
receive a TCP connection request on that port.
9.1.1.2 Subsequent Requests
A subsequent Allocate request is one received whose source and
destination IP address and ports match the internal 5-tuple of an
existing allocation. The request is processed using the general
server procedures in [1] and is processed identically to
Section 9.1.1.1, with a few important exceptions.
First, the request MUST be authenticated using the same shared secret
as the one associated with the allocation, or be authenticated using
a short term password derived from that shared secret. If the
request was authenticated but not with such a matching credential,
the server MUST generate an Allocate Error Response with a 441
response code.
Secondly, if the allocated transport address given out previously to
the client still matches the constraints in the request (in terms of
request ports, IP addresses and transport protocols), the same
allocation granted previously MUST be returned. However, if one of
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the constraints is not met any longer, because the client changed
some aspect of the request, the server MUST free the previous
allocation and allocate a new request to the client.
Finally, a subsequent Allocate request will set a new allocation
expiration timer for the allocation, effectively canceling the
previous lifetime expiration timer.
9.1.2 Client Behavior
Client behavior for Allocate requests depends on whether the request
is an initial one, for the purposes of obtaining a new relayed
transport address, or a subsequent one, used for refreshing an
existing allocation.
9.1.2.1 Initial Requests
When a client wishes to obtain a transport address, it sends an
Allocate Request to the server. This request is constructed and sent
using the general procedures defined in [1]. The server will
challenge the request for credentials. The client MAY either provide
its credentials to the server directly, or it MAY obtain a short-term
set of credentials using the Shared Secret request and then use those
as the credentials in the Allocate request.
The client SHOULD include a BANDWIDTH attribute, which indicates the
maximum bandwidth that will be used with this binding. If the
maximum is unknown, the attribute is not included in the request.
The client MAY request a particular lifetime for the allocation by
including it in the LIFETIME attribute in the request.
The client MAY include a REQUESTED-PORT-PROPS, REQUESTED-TRANSPORT,
or REQUESTED-IP attribute in the request to obtain specific types of
transport addresses. Whether these are needed depends on the
application using the relay usage. As an example, the Real Time
Transport Protocol (RTP) [5] requires that RTP and RTCP ports be an
adajacent pair, even and odd respectively, for compatibility with a
previous version of that specification. The REQUESTED-PORT-PROPS
attribute allows the client to ask the relay for those properties.
The client MUST NOT request TCP transport in an Allocate request sent
to the STUN relay server over UDP.
Processing of the response follows the general procedures of [1]. A
successful response will include both a RELAY-ADDRESS and an XOR-
MAPPED-ADDRESS attribute, providing both a relayed transport address
and a reflexive transport address, respectively, to the client. The
server will expire the allocation after LIFETIME seconds have passed
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if not refreshed by another Allocate request. The server will allow
the user to send and receive no more than the amount of data
indicated in the BANDWIDTH attribute.
If the response is an error response and contains a 442, 443 or 444
error code, the client knows that its requested properties could not
be met. The client MAY retry with different properties, with the
same properties (in a hope that something has changed on the server),
or give up, depending on the needs of the application. However, if
the client retries, it SHOULD wait 500ms, and if the request fails
again, wait 1 second, then 2 seconds, and so on, exponentially
backing off.
9.1.2.2 Subsequent Requests
Before 3/4 of the lifetime of the allocation has passed (the lifetime
of the allocation is conveyed in the LIFETIME attribute of the
Allocate Response), the client SHOULD refresh the allocation with
another Allocate Request if it wishes to keep the allocation.
To perform a refresh, the client generates an Allocate Request as
described in Section 9.1.2.1. If the initial request was
authenticated with a shared secret P that the client holds with the
server, or using a short term password derived from P through a
Shared Secret request, the client MUST use shared secret P, or a
short-term password derived from it, in the subsequent request.
In a successful response, the RELAY-ADDRESS contains the same
transport address as previously obtained, indicating that the binding
has been refreshed. The LIFETIME attribute indicates the amount of
additional time the binding will live without being refreshed. Note
that an error response does not imply that the binding has been
expired, just that the refresh has failed.
If a client no longer needs a binding, it SHOULD tear it down. If
the client wishes to explicitly remove the allocation because it no
longer needs it, it generates a subsequent Allocate request, but sets
the LIFETIME attribute to zero. This will cause the server to remove
the allocation. For TCP, the client can also remove the binding by
closing connection with the STUN relay server.
9.2 Set Active Destination Request9.2.1 Server Behavior
The Set Active Destination Request is used by a client to set an
existing external binding that will be used as the forwarding
destination of all data that is not encapsulated in STUN Send
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Indications. In addition, all data received from that external
client will be forwarded to the STUN client without encapsulation in
a Data Indication.
Once the server has identified a request as a Set Active Destination
request, the server verifies that it has arrived with a source and
destination transport address that matches the internal remote and
local transport address of an internal 5-tuple associated with an
existing allocation. If there is no matching allocation, the server
MUST generate a 437 (No Binding) Send Error Response.
The request MUST be authenticated using the same shared secret as the
one associated with the allocation, or be authenticated using a short
term password derived from that shared secret. If the request was
authenticated but not with such a matching credential, the server
MUST generate an error response with a 441 response code.
[[OPEN ISSUE: Can we eliminate the whole race condition, by requiring
the client to close the binding and wait 5 seconds (with no server
verification of this requirement) before issuing a new Set Active
Destination request? Proposed text follows:]] If an active
destination is already set, the Set Active Destination request is
rejected with a 439 Active Destination Already Set error response.
If the Set Active Destination request contains a REMOTE-ADDRESS
attribute, the IP address contained within it is added to the
permissions for this allocation, if it was not already present.
[[OPEN ISSUE: When do you ever want to set active for a destination
you have never sent to?]]
Unfortunately, there is a race condition associated with the active
destination concept. Consider the case where the active destination
is set, and the server is relaying packets towards the client. The
client knows the IP address and port where the packets came from -
the current value of the active destination. The client issues a Set
Active Destination Request to change the active destination, and
receives a response. A moment later, a data packet is received, not
encapsulated in a STUN Data Indication. What is the source if this
packet? Is it the active destination that existed prior to the Set
Active Destination request, or the one after? If the transport
between the client and the STUN server is not reliable, there is no
way to know.
To deal with this problem, a small state machine is used to force a
"cooldown" period during which the server will not relay packets
towards the client without encapsulating them. This cooldown period
gives enough time for the client to be certain that any old data
packets have left the network. Once the cooldown period ends, the
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request, but the request contained no REMOTE-ADDRESS attribute, the
state machine stays in the same state. The request is responded to
with a Set Active Destination Response. If, however, the Set Active
Destination request contained a REMOTE-ADDRESS, the server sets the
active destination to the transport address from the REMOTE-ADDRESS
attribute, and enters the "Set" state. The request is responded to
with a Set Active Destination Response. In this state, the server
will relay packets from that transport address towards the client in
unencapsulated form.
If the server receives another Set Active Destination request while
in this state, and the REMOTE-ADDRESS is present, but has a value
equal to the current active destination, the request causes no
change. The request is responded to with a Set Active Destination
Response. If, however, the request contained a REMOTE-ADDRESS which
did not match the existing active destination, or omitted the active
destination, the server enters the "transitioning" state. The
request is responded to with a Set Active Destination Response. In
this state, the server will forward all packets to the client in
encapsulated form. In addition, when this state is entered, the
client sets a timer to fire in Ta seconds. If the connection between
the client and server is unreliable, this timer SHOULD be
configurable. It is RECOMMENDED that it be set to three seconds. If
the connection between the client and server is reliable, the timer
SHOULD be set to 0 seconds, causing it to fire immediately. This
makes the transitioning state transient for reliable transports. The
value of the timer used by the server, regardless of the transport
protocol, MUST be included in a TIMER-VAL attribute in the Set Active
Destination response.
If, while in the "transitioning" state, the server receives a Set
Active Destination Request, it generates a Set Active Destination
Error Response that includes a 439 (Transitioning) response code.
Once the timer fires, the server transitions to the "Set" state if
the Set Active Destination request that caused the server to enter
"transitioning" had contained the REMOTE-ADDRESS. In this case, the
active destination is set to this transport address. If the Set
Active Destination request had not contained a REMOTE-ADDRESS
attribute, the server enters the "Not Set" state and sets the active
destination to null.
9.2.2 Client Behavior
The Set Active Destination address allows the client to create an
optimized relay function between it and the server. When the server
receives packets from a particular preferred external client, the
server will forward those packets towards the client without
encapsulating them in a Data Indication. Similarly, the client can
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send non-STUN packets to the server without encapsulation, and these
are forwarded to the external client. Sending and receiving data in
unencapsulated form is critical for efficiency purposes. One of the
primary use cases for the STUN relay usage is in support of Voice
over IP (VoIP), which uses very small UDP packets to begin with. The
extra overhead of an additional layer of encapsulation is considered
unacceptable.
The Set Active Destination request is used by the client to provide
the identity of this preferred external client. The request also has
the side effect of adding a permission for the target of the REMOTE-
ADDRESS. [[OPEN ISSUE: is this necessary?]]
The Set Active Destination address MAY contain a REMOTE-ADDRESS
attribute. This attribute, when present, provides the address of the
preferred external client to the server. When absent, it clears the
value of the preferred external client.
[[OPEN ISSUE: Proposed wording to eliminate the Set Active
Destination transitioning state machine follows.]] The client MUST
NOT send a Set Active Destination request with a REMOTE-ADDRESS
attribute over an unreliable link (ex: UDP) if an active destination
is already set for that allocation. If the client wishes to set a
new active destination, it MUST wait until 5 seconds after a
successful response is received to a Set Destination Request removing
the active destination. Failure to wait could cause the client to
receive and attribute late data forwarded by the STUN relay server to
the wrong peer.
In order for the client to know where incoming non-STUN packets were
sent from, and to be sure where non-STUN packets sent to the server
will go to, it is necessary to coordinate the value of the active
destination between the client and the server. As discussed above,
there is a race condition involved in this coordination which
requires a state machine to execute on both the client and the
server.
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Destination request prior to the receipt of a response to the
previous. The state machine will further limit the transmission of
subsequent Set Active Destination requests.
If, while in the "None Set" state, the client sent a Set Active
Destination request without a REMOTE-ADDRESS, and got a successful
response, there is no change in state. If a successful response was
received, but there was a REMOTE-ADDRESS in the request, the state
machine transitions to the "Set" state, and the client sets the
active destination to the value of the REMOTE-ADDRESS attribute that
was in the request.
If, while in the "Set" state, the client sends a Set Active
Destination request and received a 439 response, it means that there
was a temporal misalignment in the states between client and server.
The client thought that the active destination was updated on the
server, but the server was still in its transitioning state. When
this error is received, the client remains in the "Set" state. The
client SHOULD retry its Set Active Destination request, but no sooner
than 500ms after receipt of the 439 response. In addition, if, while
in the "Set" state, the client sends a Set Active Destination request
whose REMOTE-ADDRESS attribute equals the current active destination,
and that request generates a success response, the client remains in
the "Set" state.
However, if, while in the "Set" state, the client sends a Set Active
Destination request whose REMOTE-ADDRESS was either absent or not
equal to the current active destination, and receives a success
response, the client enters the "Transitioning" state. While in this
state, the client MUST NOT send a new Set Active Destination request.
The value of the active destination remains unchanged. In addition,
the client sets a timer. This timer MUST have a value equal to the
value of the TIMER-VAL attribute from the Set Active Destination
response. This is necessary for coordinating the state machines
between client and server.
Once the timer fires, if the REMOTE-ADDRESS was not absent from the
Set Active Destination request which caused the client to start the
timer, the client moves back to the "Set" state, and then updates the
value of the active destination to the value of REMOTE-ADDRESS. If
REMOTE-ADDRESS was absent, the client sets the active destination to
null and enders the "None Set" state.
9.3 Connect Request
The Connect Request is used by a client when it has obtained an
allocated transport address that is TCP. The client can use the
Connect Request to ask the server to open a TCP connection to a
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specified destination address included in the request.
9.3.1 Server Behavior
Once the server has identified a request as a Connect Request, the
server verifies that it has arrived over the connection identified by
the internal 5-tuple associated with an existing allocation. If
there is no matching allocation, the server MUST generate a 437 (No
Binding) Send Error Response.
The request MUST be authenticated using the same shared secret as the
one associated with the allocation, or be authenticated using a short
term password derived from that shared secret. If the request was
authenticated but not with such a matching credential, the server
MUST generate an error response with a 441 response code.
If the allocation is not for TCP, the server MUST reject the request
with a 445 (Operation for TCP Only) response.
If the request does not contain a REMOTE-ADDRESS attribute, the
server sends a 400 (Bad Request) Connect error response,.
If the request contains a REMOTE-ADDRESS attribute, the IP address
contained within it is added to the permissions for this allocation,
if it was not already present. This happens regardless of whether
the subsequent TCP connection attempt succeeds or not.
The server then checks to see if it has any TCP connections in
existence from the allocated transport address to the IP address and
port in DESTINATION-ADDRESS. If it does, the server responds to the
request with a Connect response, indicating to the client that a
connection exists already. confused. would only happen if another
allocation requested same destination. or if same client previously
requested connection
Next, the server attempts to open a TCP connection from the allocated
transport address to the IP address and port in the DESTINATION-
ADDRESS attribute. If the connection succeeds, the server generates
a Connect Response. If the connection attempt fails or times out,
the server generates a Connect Error Response and includes an error
response of 446 (Connection Failure). If the connection attempt is
still pending prior to the the timeout of the STUN transaction, the
server MUST send a 447 (Connection Timeout) error response. However,
the server continues to wait for the connection to get set up. If it
succeeds, the client holds on to the connection. The client can
retry the request at a later time, and if the connection has been
succesfully setup, it will result in a Success Response as described
above. no, sends immediate response. generates connect status
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messages as things are tried.
9.4 Connection Status Indication
TODO: Expand this text.
When the STUN relay to peer leg is TCP, the STUN relay client needs
to be aware of the status of these TCP connections. The STUN relay
extension defines application states for a TCP connection as follows:
LISTEN, ESTABLISHED, CLOSED. Consequently, the STUN relay server
sends a ConnectionState Indication for a binding whenever the relay
connection status changes for one of the client's bindings except
when the status changes due to a STUN relay client request (ex: an
explicit binding close or deallocation).
A STUN relay can only relay to a peer over TCP if the client
communicates with the server over TCP or TLS over TCP. Because of
this, the server can be assured that Connection Status Indications
are received reliably.
9.5 Send Indication9.5.1 Server Behavior
A Send Indication is sent by a client after it has completed its
Allocate transaction, in order to create permissions in the server
and send data to an external client.
Once the server has identified a message as a Send Indication, the
server verifies that it has arrived with a source and destination
transport address that matches the internal remote and local
transport address of an internal 5-tuple associated with an existing
allocation. If there is no matching allocation, the indication is
discarded. If there was no REMOTE-ADDRESS, the indication is
discarded. If there was no DATA attribute, the indication is
discarded.
Note that Send Indications are not authenticated and do not
contain a MESSAGE-INTEGRITY attribute. Just like non-relayed data
sent over UDP or TCP, the authenticity and integrity of this data
can only be assured using security mechanisms at higher layers.
The server takes the contents of the DATA attribute present in the
indication. If the allocation was a UDP allocation, the server
creates a UDP packet whose payload equals that content. The server
sets the source IP address of the packet equal to the allocated
transport address. The destination transport address is set to the
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contents of the REMOTE-ADDRESS attribute. The server then sends the
UDP packet. Note that any retransmissions of this packet which might
be needed are not handled by the server. It is the clients
responsibility to generate another Send indication if needed. If the
STUN relay client hasn't previously sent to this destination IP
address and port, an external 5-tuple is instantiated in the server.
Its local and remote transport addresses, respectively, are set to
the source and destination transport addresses of the UDP packet.
The server then adds the IP address of the REMOTE-ADDRESS attribute
to the permission list for this allocation.
In the case of a TCP allocation, the server checks if it has an
existing TCP connection open from the allocated transport address to
the address in the REMOTE-ADDRESS attribute. If so, the server
extracts the content of the DATA attribute and sends it on the
matching TCP connection. If the server doesn't have an existing TCP
connection to the destination, it adds the REMOTE-ADDRESS to the
permission list and discards the data. The peer must first open a
TCP connection to the STUN relay server before it can receive data
sent by the client.
9.5.2 Client Behavior
Before receiving any UDP or TCP data, a client has to send first.
Prior to the establishment of an active destination, or while the
client is in the transitioning state, transmission of data towards a
peer through the relay is done using the Send Indication. Indeed, if
the client is in the transitioning state, and it wishes to send data
through the relay, it MUST use a Send indication.
For TCP allocated transport addresses, the client needs to wait for
the peer to open a connection to the STUN relay server before it can
send data. Data sent with a Send request prior to the opening of a
TCP connection is discarded silently by the server.
The Send Indication MUST contain a REMOTE-ADDRESS attribute, which
contains the IP address and port that the data is being sent to. The
DATA attribute MAY be present, and contains the data that is to be
sent towards REMOTE-ADDRESS. If absent, the server will send an
empty UDP packet in the case of UDP. In the case of TCP, the server
will do nothing.
Since Send is an Indication, it generates no response. The client
must rely on application layer mechanisms to determine if the data
was received by the peer.
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Note that Data Indications are not authenticated and do not
contain a MESSAGE-INTEGRITY attribute. Just like non-relayed data
sent over UDP or TCP, the authenticity and integrity of this data
can only be assured using security mechanisms at higher layers.
9.6.1 Server Behavior
A server MUST send data packets towards the client using a Data
Indication under the conditions described in Section 13.1. Data
Indications MUST contain a DATA attribute containing the data to
send, and MUST contain a REMOTE-ADDRESS attribute indicating where
the data came from.
9.6.2 Client Behavior
Once a client has obtained an allocation and created permissions for
a particular external client, the server can begin to relay packets
from that external client towards the client. If the external client
is not the active destination, this data is relayed towards the
client in encapsulated form using the Data Indication.
The Data Indication contains two attributes - DATA and REMOTE-
ADDRESS. The REMOTE-ADDRESS attribute indicates the source transport
address that the request came from, and it will equal the external
remote transport address of the external client. When processing
this data, a client MUST treat the data as if it came from this
address, rather than the stun server itself. The DATA attribute
contains the data from the UDP packet or TCP segment that was
received. Note that the STUN relay server will not retransmit this
indication over UDP.
10. New Attributes
The STUN relay usage defines the following new attributes:
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0x000D: LIFETIME
0x0010: BANDWIDTH
0x0012: REMOTE-ADDRESS
0x0013: DATA
0x0016: RELAY-ADDRESS
0x0018: REQUESTED-PORT-PROPS
0x0019: REQUESTED-TRANSPORT
0x0022: REQUESTED-IP
0x0021: TIMER-VAL
10.1 LIFETIME
The lifetime attribute represents the duration for which the server
will maintain an allocation in the absence of data traffic either
from or to the client. It is a 32 bit value representing the number
of seconds remaining until expiration.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
10.2 BANDWIDTH
The bandwidth attribute represents the peak bandwidth, measured in
kbits per second, that the client expects to use on the binding. The
value represents the sum in the receive and send directions.
[[Editors note: Need to define leaky bucket parameters for this.]]
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bandwidth |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
10.3 REMOTE-ADDRESS
The REMOTE-ADDRESS specifies the address and port of the peer as seen
from the STUN relay server. It is encoded in the same way as MAPPED-
ADDRESS.
10.4 DATA
The DATA attribute is present in Send Indications and Data
Indications. It contains raw payload data that is to be sent (in the
case of a Send Request) or was received (in the case of a Data
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Indication).
10.5 RELAY-ADDRESS
The RELAY-ADDRESS is present in Allocate responses. It specifies the
address and port that the server allocated to the client. It is
encoded in the same way as MAPPED-ADDRESS.
10.6 REQUESTED-PORT-PROPS
This attribute allows the client to request certain properties for
the port that is allocated by the server. The attribute can be used
with any transport protocol that has the notion of a 16 bit port
space (including TCP and UDP). The attribute is 32 bits long. Its
format is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved = 0 |B| A |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The lower two bits (labeled A in the diagram) are for requested port
alignment.
00 no specific port alignment
01 odd port number
10 even port number
11 even port number; reserve next higher port
The next higher bit (labeled B in the diagram) is for requested port
allocation behavior.
0 no special behavior requested
1 specific port requested
All other bits in this attribute are reserved and MUST be set to
zero.
Even Port is a request to the server to allocate a port with even
parity. The port filter is not used with this property. Odd Port is
a request to the server to allocate a port with odd parity. The port
filter is not used with this property. Even port, with a hold on the
next higher port, is a request to the server to allocate an even
port. Furthermore, the client indicates that it will want the next
higher port as well. As such, the client requests that the server,
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if it can, not allocate the next higher port to anyone unless that
port is explicitly requested, which the client will itself do. The
port filter is not used with this property. Finally, the Specific
Port property is a request for a specific port. The port that is
requested is contained in the Port filter.
Extensions to the relay usage can define additional port properties.
[[TODO: Add IANA registry]]
10.7 REQUESTED-TRANSPORT
This attribute is used by the client to request a specific transport
protocol for the allocated transport address. It is a 32 bit
unsigned integer. Its values are:
0x0000 0000: UDP
0x0000 0001: TCP
If an Allocate request is sent over TCP and requests a UDP
allocation, or an Allocate request is sent over TLS over TCP and
requests a UDP or TCP allocation, the server will relay data between
the two transports.
Extensions to the relay usage can define additional transport
protocols. [[TODO: Add IANA registry]]
10.8 REQUESTED-IP
The REQUESTED-IP attribute is used by the client to request that a
specific IP address be allocated to it. This attribute is needed
since it is anticipated that STUN relays will be multi-homed so as to
be able to allocate more than 64k transport addresses. As a
consequence, a client needing a second transport address on the same
interface as a previous one can make that request.
The format of this attribute is identical to MAPPED-ADDRESS.
However, the port component of the attribute is ignored by the
server. If a client wishes to request a specific IP address and
port, it uses both the REQUESTED-IP and REQUESTED-PORT-PROPS
attributes.
10.9 TIMER-VAL
The TIMER-VAL attribute is used only in conjunction with the Set
Active Destination response. It conveys from the server, to the
client, the value of the timer used in the server state machine.
Coordinated values are needed for proper operation of the mechanism.
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The attribute is a 32 bit unsigned integer representing the number if
milliseconds used by the server for its timer. [TODO delete this
attribute if we get rid of the timer]
11. New Error Response Codes
The STUN relay usage defines the following new Error response codes:
437 (No Binding): A request was received by the server that
requires an allocation to be in place. However, there is none yet
in place.
439 (Transitioning): A Set Active Destination request was received
by the server. However, a previous request was sent within the
last few seconds, and the server is still transitioning to that
active destination. Please repeat the request later.
441 (Wrong Username): A STUN relay request was received for an
allocated binding, but it did not use the same username and
password that were used in the allocation. The client must supply
the proper credentials, and if it cannot, it should teardown its
binding, allocate a new one time password, and try again.
442 (Unsupported Transport Protocol): The Allocate request asked
for a transport protocol to be allocated that is not supported by
the server.
443 (Invalid IP Address): The Allocate request asked for a
transport address to be allocated from a specific IP address that
is not valid on the server.
444 (Invalid Port): The Allocate request asked for a port to be
allocated that is not available on the server.
445 (Operation for TCP Only): The client tried to send a request
to perform a TCP-only operation on an allocation, and allocation
is UDP.
446 (Connection Failure): The attempt by the server to open the
connection failed.
447 (Connection Timeout): The attempt by the server to open the
connection could not be completed, and is still in progress.
12. Client ProceduresRosenberg, et al. Expires April 9, 2007 [Page 32]

Internet-Draft TURN October 200612.1 Receiving and Sending Unencapsulated Data
Once the active destination has been set, a client will receive both
STUN and non-STUN data on the socket on which the Allocate Request
was sent. The encapsulation behavior depends on the transport
protocol used between the STUN client and the STUN relay server.
12.1.1 Datagram Protocols
If the allocation was over UDP, datagrams which contain the STUN
magic cookie are treated as STUN requests. All other data is non-
STUN data, which MUST be processed as if it had a source IP address
and port equal to the value of the active destination.
If the client wants to send data to the peer which contains the magic
cookie in the same location as a STUN request, it MUST send that data
encapsulated in a Send Indication, even if the active destination is
set.
In addition, once the active destination has been set, if the client
is in the "Set" state, it MAY send data to the active destination by
sending data on that same socket. Unencapsulated data MUST NOT be
sent while in the "Not Set" or "Transitioning" states. However, it
is RECOMMENDED that the client not send unencapsulated data for
approximately 500 milliseconds after the client enters the "Set"
state. This eliminates any synchronization problems resulting from
network delays. Of course, even if the active destination is set,
the client can send data to that destination at any time by using the
Send Indication.
12.1.2 Stream Transport Protocols
If the allocation was over TCP or TLS over TCP, the client will
receive data framed as described in Section 8. The client MUST treat
data encapsulated as data with this framing as if it originated from
the active destination.
For the STUN relay usage, the client always send data encapsulated
using this framing scheme. It SHOULD frame data to the active
destination as data or it MAY place the data inside a Send
Indications and frame this as STUN traffic. [TODO make this more
clear]
13. Server Procedures
Besides the processing of the request and indications described
above, this specification defines rules for processing of data
packets received by the STUN server. There are two cases - receipt
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of any packets on an allocated address, and receipt of non-STUN data
on its internal local transport address.
13.1 Receiving Data on Allocated Transport Addresses13.1.1 TCP Processing
If a server receives a TCP connection request on an allocated TCP
transport address, it checks the permissions associated with that
allocation. If the source IP address of the TCP SYN packet match one
of the permissions, the TCP connection is accepted. Otherwise, it is
rejected. No information is passed to the client about the
acceptance of the connection; rather, data passed to the client with
a source transport address it has not seen before serves this
purpose. [[TODO fix]]
If a server receives data on a TCP connection that terminates on the
allocated TCP transport address, the server checks the value of the
active destination. If it equals the source IP address and port of
the data packet (in other words, if the active destination identifies
the other side of the TCP connection), the server checks the state
machine of the allocation. If the state is "Set", the data is taken
from the TCP connection and sent towards the client in unencapsulated
form. Otherwise, the data is sent towards the client in a Data
Indication, also known as encapsulated form. In this form, the
server MUST add a REMOTE-ADDRESS which corresponds to the external
remote transport address of the TCP connection, and MUST add a DATA
attribute containing the data received on the TCP connection.
Sending of the data towards the client, whether in encapsulated or
unencapsulated form, depends on the linkage with the client. If the
linkage with the client is over UDP, the data is placed in a UDP
datagram and sent over the linkage. Note that the server will not
retransmit this data to ensure reliability. If the linkage with the
client is over TCP, the data is placed into the TCP connection
corresponding to the linkage. If the TCP connection generates an
error (because, for example, the incoming TCP packet rate exceeds the
throughput of the TCP connection to the client), the data is
discarded silently by the server.
Note that, because data is forwarded blindly across TCP bindings, TLS
will successfully operate over a STUN relay allocated TCP port if the
linkage to the client is also TCP.
13.1.2 UDP Processing
If a server receives a UDP packet on an allocated UDP transport
address, it checks the permissions associated with that allocation.
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If the source IP address of the UDP packet matches one of the
permissions, the UDP packet is accepted. Otherwise, it is discarded.
Assuming the packet is accepted, it must be forwarded to the client.
It will be forwarded in either encapsulated or unencapsulated form.
To determine which, the server checks the value of the active
destination. If it equals the source IP address and port of the UDP
packet, the server checks the state machine of the allocation. If
the state is "Set", the data is taken from the UDP payload and sent
towards the client in unencapsulated form. Otherwise, the data is
sent towards the client in a Data Indication, also known as
encapsulated form. In this form, the server MUST add a REMOTE-
ADDRESS which corresponds to the external remote transport address of
the UDP packet, and MUST add a DATA attribute containing the data
payload of the UDP packet.
Sending of the data towards the client, whether in encapsulated or
unencapsulated form, depends on the linkage with the client. If the
linkage with the client is over UDP, the data is placed in a UDP
datagram and sent over the linkage. Note that the server will not
retransmit this data to ensure reliability. If the linkage with the
client is over TCP, the data is placed into the TCP connection
corresponding to the linkage. If the TCP connection generates an
error (because, for example, the incoming UDP packet rate exceeds the
throughput of the TCP connection), the data is discarded silently by
the server.
13.2 Receiving Data on Internal Local Transport Addresses
If a server receives a UDP packet from the client on its internal
local transport address, and it is coming from an internal remote
transport address associated with an existing allocation, it
represents UDP data that the client wishes to forward. If the active
destination is not set, the server MUST discard the packet. If the
active destination is set, and the allocated transport protocol is
TCP, the server selects the TCP connection from the allocated
transport address to the active destination. The data is then sent
over that connection. If the transmission fails due to a TCP error,
the data is discarded silently by the server. If the active
destination is set, and the allocated transport protocol is UDP, the
server places the data from the client in a UDP payload, and sets the
destination address and port to the active destination. The UDP
packet is then sent with a source IP address and port equal to the
allocated transport address. Note that the server will not
retransmit the UDP datagram.
If a server receives data on a TCP connection to a client, the server
retrieves the allocation bound to that connection. If the active
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destination for the allocation is not set, the server MUST discard
the data. If the active destination is set, and the allocated
transport protocol is TCP, the server selects the TCP connection from
the allocated transport address to the active destination. The data
is then sent over that connection. If the transmission fails due to
a TCP error, the data is discarded silently by the server. If the
active destination is set, and the allocated transport protocol is
UDP, the server places the data from the client in a UDP payload, and
sets the destination address and port to the active destination. The
UDP packet is then sent with a source IP address and port equal to
the allocated transport address. Note that the server will not
retransmit the UDP datagram.
If a TCP connection from a client is closed, the associated
allocation is destroyed. This involves terminating any TCP
connections from the allocated transport address to external clients
(applicable only when the allocated transport address was TCP), and
then freeing the the allocated transport address (and all associated
state maintained by the server) for use by other clients.
Note that the state of the allocation, whether it is "Set", "Not
Set", or "Transitioning", has no bearing on the rules for forwarding
of packets received from clients. Only the value of the active
destination is relevant.
13.3 Lifetime Expiration
When the allocation expiration timer for a binding fires, the server
MUST destroy the allocation. This involves terminating any TCP
connections from the allocated transport address to external clients
(applicable only when the allocated transport address was TCP), and
then freeing the allocated transport address (and all associated
state maintained by the server) for use by other clients.
[[OPEN ISSUE: This is a change from the previous version, which
allowed data traffic to keep allocations alive. This change was made
based on implementation considerations, as it allows an easier
separation of packet processing and signaling. Is this OK?]]
14. Security Considerations
TODO: Need to spend more time on this.
STUN servers implementing this relay usage allocate bandwidth and
port resources to clients, in constrast to the usages defined in [1].
Therefore, a STUN server providing the relay usage requires
authentication and authorization of STUN requests. This
authentication is provided by mechanisms defined in the STUN
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specification itself. In particular, digest authentication and the
usage of short-term passwords, obtained through a digest exchange
over TLS, are available. The usage of short-tem passwords ensures
that the Allocate Requests, which often do not run over TLS, are not
susceptible to offline dictionary attacks that can be used to guess
the long lived shared secret between the client and the server.
Because STUN servers implementing the relay usage allocate resources,
they can be susceptible to denial-of-service attacks. All Allocate
Requests are authenticated, so that an unknown attacker cannot launch
an attack. An authenticated attacker can generate multiple Allocate
Requests, however. To prevent a single malicious user from
allocating all of the resources on the server, it is RECOMMENDED that
a server implement a modest per user cap on the amount of bandwidth
that can be allocated. Such a mechanism does not prevent a large
number of malicious users from each requesting a small number of
allocations. Attacks as these are possible using botnets, and are
difficult to detect and prevent. Implementors of the STUN relay
usage should keep up with best practices around detection of
anomalous botnet attacks.
A client will use the transport address learned from the RELAY-
ADDRESS attribute of the Allocate Response to tell other users how to
reach them. Therefore, a client needs to be certain that this
address is valid, and will actually route to them. Such validation
occurs through the message integrity checks provided in the Allocate
response. They can guarantee the authenticity and integrity of the
allocated addresss. Note that the STUN relay usage is not
susceptible to the attacks described in Section 12.2.3, 12.2.4,
12.2.5 or 12.2.6 of RFC 3489 [[TODO: Update references once 3489bis
is more stable]]. These attacks are based on the fact that a STUN
server mirrors the source IP address, which cannot be authenticated.
STUN does not use the source address of the Allocate Request in
providing the RELAY-ADDRESS, and therefore, those attacks do not
apply.
The relay usage cannot be used by clients for subverting firewall
policies. The relay usage has fairly limited applicability,
requiring a user to send a packet to a peer before being able to
receive a packet from that peer. This applies to both TCP and UDP.
Thus, it does not provide a general technique for externalizing TCP
and UDP sockets. Rather, it has similar security properties to the
placement of an address-restricted NAT in the network, allowing
messaging in from a peer only if the internal client has sent a
packet out towards the IP address of that peer. This limitation
means that the relay usage cannot be used to run web servers, email
servers, SIP servers, or other network servers that service a large
number of clients. Rather, it facilitates rendezvous of NATted
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clients that use some other protocol, such as SIP, to communicate IP
addresses and ports for communications.
Confidentiality of the transport addresses learned through Allocate
requests does not appear to be that important, and therefore, this
capability is not provided.
Relay servers are useful even for users not behind a NAT. They can
provide a way for truly anonymous communications. A user can cause a
call to have its media routed through a STUN server, so that the
user's IP addresses are never revealed.
TCP transport addresses allocated by Allocate requests will properly
work with TLS and SSL. However, any relay addresses learned through
an Allcoate will not operate properly with IPSec Authentication
Header (AH) [11] in transport mode. IPSec ESP [12] and any tunnel-
mode ESP or AH should still operate.
15. IANA Considerations
This specification defines several new STUN messages, STUN
attributes, and STUN response codes. This section directs IANA to
add these new protocol elements to the IANA registry of STUN protocol
elements.
15.1 New STUN Requests, Responses, and Indications
0x0003 : Allocate Request
0x0103 : Allocate Response
0x0113 : Allocate Error Response
0x0004 : Send Indication
0x0115 : Data Indication
0x0006 : Set Active Destination Request
0x0106 : Set Active Destination Response
0x0116 : Set Active Destination Error Response
0x0007 : Connect Request
0x0107 : Connect Response
0x0117 : Connect Error Response
0x0118 : Connect Status Indication
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0x000D: LIFETIME
0x0010: BANDWIDTH
0x0012: REMOTE-ADDRESS
0x0013: DATA
0x0016: RELAY-ADDRESS
0x0018: REQUESTED-PORT-PROPS
0x0019: REQUESTED-TRANSPORT
0x0022: REQUESTED-IP
0x0021: TIMER-VAL
15.3 New STUN response codes
437 No Binding
439 Transitioning
441 Wrong Username
442 Unsupported Transport Protocol
443 Invalid IP Address
444 Invalid Port
445 Operation for TCP Only
446 Connection Failure
447 Connection Timeout
16. IAB Considerations
The IAB has studied the problem of ``Unilateral Self Address
Fixing'', which is the general process by which a client attempts to
determine its address in another realm on the other side of a NAT
through a collaborative protocol reflection mechanism RFC 3424 [13].
The STUN relay extension is an example of a protocol that performs
this type of function. The IAB has mandated that any protocols
developed for this purpose document a specific set of considerations.
This section meets those requirements.
16.1 Problem Definition
>From RFC 3424 [13], any UNSAF proposal must provide:
Precise definition of a specific, limited-scope problem that is to
be solved with the UNSAF proposal. A short term fix should not be
generalized to solve other problems; this is why "short term
fixes usually aren't".
The specific problem being solved by the STUN relay extension is for
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a client, which may be located behind a NAT of any type, to obtain an
IP address and port on the public Internet, useful for applications
that require a client to place a transport address into a protocol
message, with the expectation that the client will be able to receive
packets from a single host that will send to this address. Both UDP
and TCP are addressed. It is also possible to send packets so that
the recipient sees a source address equal to the allocated address.
STUN relays, by design, does not allow a client to run a server (such
as a web or SMTP server) using a STUN relay address. STUN relays are
useful even when NAT is not present, to provide anonymity services.
16.2 Exit Strategy
From [13], any UNSAF proposal must provide:
Description of an exit strategy/transition plan. The better short
term fixes are the ones that will naturally see less and less use
as the appropriate technology is deployed.
It is expected that STUN relays will be useful indefinitely, to
provide anonymity services. When used to facilitate NAT traversal,
STUN relay does not itself provide an exit strategy. That is
provided by the Interactive Connectivity Establishment (ICE) [14]
mechanism. ICE allows two cooperating clients to interactively
determine the best addresses to use when communicating. ICE uses
STUN-allocated relay addresses as a last resort, only when no other
means of connectivity exists. As a result, as NATs phase out, and as
IPv6 is deployed, ICE will increasingly use other addresses (host
local addresses). Therefore, clients will allocate STUN relay
addresses, but not use them, and therefore, de-allocate them.
Servers will see a decrease in usage. Once a provider sees that its
STUN relay servers are not being used at all (that is, no media flows
through them), they can simply remove them. ICE will operate without
STUN-allocated relay addresses.
16.3 Brittleness Introduced by STUN relays
From [13], any UNSAF proposal must provide:
Discussion of specific issues that may render systems more
"brittle". For example, approaches that involve using data at
multiple network layers create more dependencies, increase
debugging challenges, and make it harder to transition.
The STUN relay extension introduces brittleness in a few ways.
First, it adds another server element to any system, which adds
another point of failure. It requires clients to demultiplex STUN
relay packets and data based on hunting for a MAGIC-COOKIE in the
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STUN messages. It is possible (with extremely small probabilities)
that this cookie could appear within a data stream, resulting in mis-
classification. That might introduce errors into the data stream
(they would appear as lost packets), and also result in loss of a
binding. STUN relay relies on any NAT bindings existing for the
duration of the bindings held by the STUN relay server. Neither the
client nor the STUN relay server have a way of reliably determining
this lifetime (STUN can provide a means, but it is heuristic in
nature and not reliable). Therefore, if there is no activity on an
address learned from STUN for some period, the address might become
useless spontaneously.
STUN relays will result in potentially significant increases in
packet latencies, and also increases in packet loss probabilities.
That is because it introduces an intermediary on the path of a packet
from point A to B, whose location is determined by application-layer
processing, not underlying routing topologies. Therefore, a packet
sent from one user on a LAN to another on the same LAN may do a trip
around the world before arriving. When combined with ICE, some of
the most problematic cases are avoided (such as this example) by
avoiding the usage of STUN relay addresses. However, when used, this
problem will exist.
Note that STUN relay does not suffer from many of the points of
brittleness introduced by the STUN binding or discovery usages. STUN
relay will work with all existing NAT types known at the time of
writing, and for the forseeable future. STUN relay does not
introduce any topological constraints. STUN relay does not rely on
any heuristics for NAT type classification.
16.4 Requirements for a Long Term Solution
>From [13]}, any UNSAF proposal must provide:
Identify requirements for longer term, sound technical solutions
-- contribute to the process of finding the right longer term
solution.
Our experience with STUN relay continues to validate our belief in
the requirements outlined in Section 14.4 of STUN.
16.5 Issues with Existing NAPT Boxes
>From [13], any UNSAF proposal must provide:
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Discussion of the impact of the noted practical issues with
existing, deployed NA[P]Ts and experience reports.
A number of NAT boxes are now being deployed into the market which
try and provide "generic" ALG functionality. These generic ALGs hunt
for IP addresses, either in text or binary form within a packet, and
rewrite them if they match a binding. This usage avoids that problem
by using the XOR-MAPPED-ADDRESS attribute instead of the MAPPED-
ADDRESS
17. Example
In this example, a client is behind a NAT. The client has a private
address of 10.0.1.1. The STUN server is on the public side of the
NAT, and is listening for STUN relay requests on 192.0.2.3:8776. The
public side of the NAT has an IP address of 192.0.2.1. The client is
attempting to send a SIP INVITE to a peer, and wishes to allocate an
IP address and port for inclusion in the SDP of the INVITE.
Normally, STUN relays would be used in conjunction with ICE when
applied to SIP. For simplicities sake, STUN relay is showed without
ICE.
The client communicates with a SIP user agent on the public network.
This user agent uses a 192.0.2.17:12734 for receipt of its RTP
packets.
Client NAT STUN Server Peer
| | | |
|(1) Allocate | | |
|S=10.0.1.1:4334 | | |
|D=192.0.2.3:8776 | | |
|------------------>| | |
| | | |
| |(2) Allocate | |
| |S=192.0.2.1:63346 | |
| |D=192.0.2.3:8776 | |
| |------------------>| |
| | | |
| |(3) Error | |
| |S=192.0.2.3:8776 | |
| |D=192.0.2.1:63346 | |
| |<------------------| |
| | | |
|(4) Error | | |
|S=192.0.2.3:8776 | | |
|D=10.0.1.1:4334 | | |
|<------------------| | |
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| | | |
|(21) Response | | |
|S=192.0.2.3:8776 | | |
|D=10.0.1.1:4334 | | |
|<------------------| | |
| | | |
| | | |
| | | after 3s|
| | | |
| | | |
| | |(22) RTP |
| | |S=192.0.2.17:12734 |
| | |D=192.0.2.3:32766 |
| | |<------------------|
| | | |
| |(23) RTP | |
| |S=192.0.2.3:8776 | |
| |D=192.0.2.1:63346 | |
| |<------------------| |
| | | |
|(24) RTP | | |
|S=192.0.2.3:8776 | | |
|D=10.0.1.1:4334 | | |
|<------------------| | |
| | | |
| | | |
Figure 16
The call flow is shown in Figure 16. The client allocates a port
from the local operating system on its private interface, obtaining
4334. It then attempts to secure a port for RTP traffic. RTCP
processing is not shown. The client sends an Allocate request (1)
with a source address (denoted by S) of 10.0.1.1:4334 and a
destination (denoted by D) of 192.0.2.3:8776. This passes through
the NAT (2), which creates a mapping from the 192.0.2.1:63346 to the
source IP address and port of the request, 10.0.1.1:4334. This
request is received at the STUN server, which challenges it (3),
requesting credentials. This response is passed to the client (4).
The client retries the request, this time with credentials (5). This
arrives at the server (6). The request is now authenticated. The
server provides a UDP allocation, 192.0.2.3:32766, and places it into
the RELAY-ADDRESS (denoted by RA) in the response (7). It also
reflects the source IP address and port of the request into the
MAPPED-ADDRESS (denoted by MA) in the response. This passes through
the NAT to the client (8). The client now proceeds to perform a
basic SIP call setup. In message 9, it includes the relay address
into the SDP of its INVITE. The called party responds with a 200 OK,
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and includes its IP address - 192.0.2.17:12734. The exchange
completes with an ACK (11).
Next, user A sends an RTP packet. Since the active destination has
not been set, the client decides to use the Send indication. It does
so, including the RTP packet as the contents of the DATA attribute.
The REMOTE-ADDRESS attribute (denoted by DA) is set to 192.0.2.17:
12734, learned from the 200 OK. This is sent through the NAT
(message 12) and arrives at the STUN server (message 13). The server
extracts the data contents, and sends the packet towards REMOTE-
ADDRESS (message 14). Note how the source address and port in this
packet is 192.0.2.3:32766, the allocated transport address given to
the client. The act of sending the packet with Send causes the STUN
server to install a permission for 192.0.2.17.
Indeed, the called party now sends an RTP packet toward the client
(message 15). This arrives at the STUN server. Since a permission
has been set for the IP address in the source of this packet, it is
accepted. As no active destination is set, the STUN server
encapsulates the contents of the packet in a Data Indication (message
16), and sends it towards the client. The REMOTE-ADDRESS attribute
(denoted by RA) indicates the source of the packet - 192.0.2.17:
12734. This is forwarded through the NAT to the client (message 17).
The client decides to optimize the path for packets to and from
192.0.2.17:12734. So, it issues a Set Active Destination request
(message 18) with a REMOTE-ADDRESS of 192.0.2.17:12734. This passes
through the NAT and arrives at the STUN server (message 19). This
generates a successful response (message 20) which is passed to the
client (message 21). At this point, the server and client are in the
transitioning state. A little over 3 seconds later (by default), the
state machines transition back to "Set". Until this point, packets
from the called party would have been relayed back to the client in
Data Indications. Now, the next RTP packet shows up at the STUN
server (message 22). Since the source IP address and port match the
active destination, the RTP packet is relayed towards the client
without encapsulation (message 23 and 24).
18. Acknowledgements
The authors would like to thank Marc Petit-Huguenin for his comments
and suggestions.
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